The disclosure of Japanese Patent Application No. HEI 11-146793 filed on May 26, 1999 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The present invention relates to a control apparatus and a control method of a control system having a sliding resistance and, more particularly, to a control apparatus and a control method for controlling a control system having a sliding resistance by a sliding mode control method.
2. Description of the Related Art
As a technology related to the invention, there is a control apparatus that controls a hydraulic control system by using a sliding mode control method (proposed in, for example, Japanese Patent Application Laid-Open No. HET 8-61122, etc.). The sliding mode control method provides an excellent control system that has strong robustness. However, it is known that if unobservable parasitic dynamics or a long dead time exists in the control system of the sliding mode control method, the robustness of the control system remarkably decreases. To solve this problem, various methods are proposed, for example, a method in which the relationship between a switching surface "sgr" and an amount of operation u, which is essentially discontinuous as indicated in FIG. 11, is substituted with a continuous saturation function as indicated in FIG. 12 (for example, NONAMI Kenzou, and TIAN Hongqi. Sliding Mode Control. Published by Corona Sha: p.27, etc.). In another proposed method, when the deviation of an actual value from a target value in the control system is relatively small, the switching of the control gain is changed from switching in software to switching in hardware to increase the switching speed (for example, Japanese Patent Application Laid-Open No. HEI 5-88749, etc.). In yet another proposed method the gain is reduced every time a switching surface crosses a switching surface (for example, Japanese Patent Application Laid-Open No. HEI 2-75001, etc.).
However, neither the method in which the relationship between the switching surface and the amount of operation is substituted with a saturation function, nor the method in which the gain is reduced every time the switching surface crosses the switching surface, causes the feedback control system to achieve convergence because when the control system is in a nearly converged state, that is, when the output of the switching surface is close to zero, the output gain approaches zero. This is because when the feedback control system is in the nearly converged state, switching between the static friction coefficient and the dynamic friction coefficient of the sliding surface occurs.
Thus, the presence of a sliding resistance in a control system gives rise to a problem of losing the effectiveness of the sliding mode control. For example, if a simulation is performed on the assumption that the sliding resistance is 1.2 times as great as the input value that is needed in order to shift the slip speed x1 of a clutch from 0 rpm to a target value of 20 rpm and that the dead time is 128 ms, the simulation provides results as indicated in FIGS. 13 to 16. As indicated in FIG. 13, the presence of the sliding resistance prevents the slip speed x1 of the clutch from converging to the target value. FIG. 13 is a graph indicating time-dependent changes of the slip speed x1 of the clutch. FIG. 14 is a graph indicating time-dependent changes of the differential x2 of the slip speed x1. FIG. 15 is a graph indicating time-dependent changes of the amount of operation u. FIG. 16 is a graph indicating time-dependent changes of the switching surface "sgr".
Accordingly, it is an object of the invention to provide a control apparatus of a control system having a sliding resistance that is able to retain the robustness of the sliding mode control. It is another object of the invention to provide a control apparatus of a control system having a sliding resistance that is able to prevent deterioration of the controlling performance due to the presence of the resistance.
In order to achieve at least one of the aforementioned and other objects of the invention, the control apparatus of a control system having a sliding resistance of the invention has a construction as follows. A control apparatus according to one aspect of the invention controls a control system having a sliding resistance by a sliding mode control method. The control apparatus includes a switching surface computing device for computing a switching surface based on a quantity of state and a target value, a carrier wave adding device for adding a carrier wave having a waveform that has a gradient within a predetermined range to the switching surface computed by the switching surface computing device, and an operation amount setting device for setting an amount of operation based on a value of a switching surface obtained by adding the carrier wave to the switching surface computed by the switching surface computing device.
In the control apparatus of a control system having a sliding resistance according to the invention, the value of the switching surface changes periodically due to the carrier wave added to the switching surface, even when the feedback control system reaches a nearly converged state and, therefore, the value of output of the switching surface is in proximity of zero. Therefore, even through the control system has a sliding resistance, the control apparatus is able to control the control system in a good manner. Furthermore, since the amount of operation is changed in accordance with the value of the switching surface for the control, high robustness can be maintained.
A control method according to another aspect of the invention computes a switching surface based on a quantity of state and a target value, adds a carrier wave having a waveform that has a gradient within a predetermined range to the switching surface computed, and sets an amount of operation based on a value of a switching surface obtained by adding the carrier wave to the computed switching surface.
The xe2x80x9cwaveform that has a gradient within a predetermined rangexe2x80x9d does not include a waveform that has an infinite gradient at a vertically changing portion of the waveform, for example, a rectangular waveform such as a dither waveform or the like, but includes various waveforms that can be expressed by periodic functions, for example, sine waveforms having finite gradients, composite waveforms of a plurality of sine waveforms, sawtooth pulse waveforms, trapezoidal waveforms, and the like. Furthermore, since the xe2x80x9cwaveform that has a gradient within a predetermined rangexe2x80x9d is the waveform of the carrier wave, the xe2x80x9cwaveformxe2x80x9d is limited to waveforms that periodically change.